1. Field of the Invention
This invention is directed to laminates comprising polyester and paperboard and to a process for forming such laminates. More specifically, this invention is directed to polyester/paperboard laminates which include an intermediate adhesive layer and to a method of forming such laminates by coextruding a layer of polyester and a layer of an adhesive onto a paperboard substrate.
2. Description of the Prior Art
The use of polyester/paperboard laminates in the preparation of ovenable containers is increasing. However, due to the crystalline nature and non-adherent characteristics of the polyesters, particularly polyethylene terephthalate, fabrication of such laminates has been accomplished only with difficulty, requiring special treatment of the paperboard substrate.
A process for preparing a polyester/paperboard laminate is disclosed in U.S. Pat. No. 3,939,025 to Kane, wherein the paperboard substrate is heated to a temperature of at least 285.degree. F. and is thereafter extrusion coated with polyester material. Another process for adhering polyester to paperboard involves pretreating a paperboard substrate by exposing it to electrical discharge and thereafter extrusion coating the pretreated substrate.
Even when such paperboard pretreatment processes are used, however, it has often been necessary to extrude polyethylene terephthalate at a 26 to 28 pound per ream coat weight in order to obtain adequate adherence of the polyester to the paperboard. However, general performance requirements for the laminate as used in, for example, ovenable containers, do not require such a heavy weight coating. Accordingly, the excess polyester which must be used places an economic burden on the system.
3. Summary of the Invention
This invention provides a process for the preparation of a laminate comprising a layer of a polyester, particularly polyethylene terephthalate, and a paperboard layer, which process comprises coextruding a polyester layer and a polymeric adhesive layer which adheres to both paperboard and polyester, onto a paperboard substrate. By use of this process, polyester/paperboard laminates can be prepared using as little as 10 pounds of polyester per ream without delamination. The adhesive layer can be pigmented thereby providing visual enhancement of the laminate product. Moreover, the paperboard can be treated with sizing agents or the like without serious adverse effects on the adherence of the polyester layer. The polyester/paperboard laminates produced in accordance with this invention are less costly than those of the prior art since they require a smaller coat weight of polyester.